1. Field of the Invention
The present invention relates: to a dispersion of a colorant being suitably used in the production of a magenta toner for developing electrostatic images wherein the toner is superior, for example, in light transmitting property and coloring ability and has a wide color gamut and can be employed suitably in image formation by electrophotography; to a toner for developing electrostatic images obtained using the dispersion; to a method for efficiently producing the toner for developing electrostatic images; and to an electrostatic image developer and image-forming method using the toner for developing electrostatic images.
2. Description of the Related Art
Methods for making image information visible via electrostatic images, such as electrophotography, are widely used in various fields. In electrophotography, an electrostatic image is formed on a photosensitive body via a charging step, an exposure step and the like. The electrostatic image is developed by use of a developer containing toner particles. Thus, the electrostatic image is made visible via a transfer step and a fixation step.
As the developer to be used here, two-component developers, which include toner particles and carrier particles, and mono-component developers, which include magnetic toner particles or nonmagnetic toner particles, are known.
The toner particles in these developers are generally produced by a kneading-pulverizing process. The kneading-pulverizing process is a method in which desired toner particles are produced by melt-kneading a thermoplastic resin and the like with a pigment, a charge-controlling agent, a releasing agent such as wax and the like, finely pulverizing the melt-kneaded material after cooling, and classifying the pulverized matter. In addition, inorganic and/or organic fine particles may, if necessary, be added to the surface of the toner particles produced by the kneading-pulverizing process for the purpose of improving fluidity, cleanability and the like.
Generally the toner particles produced by the kneading-pulverizing process are irregularly shaped and are not uniform in surface composition. Depending on the pulverizability of the material to be used and on the conditions of the pulverization step, the shape and surface composition of toner particles change slightly. However, it is difficult to control these factors intentionally at desired degrees. In particular, in the case of toner particles produced by the kneading-pulverizing process using a highly pulverizable material, the toner particles are often further micronized or their shape changed inside a developing device due to mechanical forces such as various types of shearing force. As a result, the following problems have arisen. Regarding two-component developers, micronized toner particles may stick on the surface of a carrier and accelerate the electrostatic degradation of the developer. On the other hand, regarding mono-component developers, their particle size distribution is enlarged, thus fine toner particles might be scatter and a reduction in developability with a change of the toners shape might cause deterioration of image quality.
Irregularly shaped toner particles cannot demonstrate a sufficient fluidity even if a fluidity aid is added thereto. Moreover, fine particles of the fluidity aid are conveyed to recessed portions of the toner particles by the mechanical forces, such as shearing force, applied during the use of the toner and the fine particles become buried in the recessed portions. Thus, problems, such as reduction in fluidity with time and deteriorations of developability, transferability and cleanability arise. In addition, when such a toner is reused after being recovered through a cleaning treatment and fed back to a developing device, deterioration of image quality easily occurs. Increasing the amount of the fluidity aid is a possible way to prevent these problems. This, however, might cause another problem of generation of black points on a photosensitive body and a problem of scattering particles of the fluidity aid.
On the other hand, in the case of toners containing a releasing agent, such as wax, which has been internally added, the releasing agent may be exposed in the surface of toner particles when it is used in combination with some thermoplastic resin. Particularly in the case of toners including a combination of a resin which has been imparted with elasticity by a high molecular weight component and which is slightly resistant to pulverization and a fragile wax such as polyethylene, much exposure of polyethylene can be seen in the surface of the toner particles. Such toners are advantageous from the viewpoints of mold releasability at the time of fixation and ease of cleaning untransferred toner. However, the polyethylene on the surface of the toner particles are easily detached from the toner particles by action of mechanical force, such as shearing force, applied in a developing device and migrate to a developing roll, a photosensitive body, a carrier and the like. Thus, these elements are easily contaminated to cause reduction in reliability as a developer. Under such circumstances, an emulsion-polymerization flocculation process has been proposed in Japanese Patent Application Laid-Open (JP-A) Nos. 63-282752 and 6-250439 as means for producing a toner including particles whose shape and surface composition are intentionally controlled. The emulsion-polymerization flocculation process is a process as follows. A resin dispersion is prepared by emulsion polymerization. Separately, a colorant dispersion in which a colorant is dispersed in solvent is prepared. These are mixed to form aggregated particles with a size corresponding to the toner particle diameter. Then the aggregated particles are heated and fused together. Thus toner particles are obtained. When the emulsion-polymerization flocculation process is used, the shape of toner can be controlled optionally from an irregular form through a spherical form through a choice of the heating temperature. However, in the emulsion-polymerization flocculation process, because aggregated particles in a uniformly mixed state are fused together, the composition from the inside of the toner to the surface thereof becomes uniform. Therefore, it is difficult to intentionally control the structure and composition of the surface of toner particles. In particular, when the aggregated particles contain a releasing agent, the releasing agent is present in the surface of the toner particles after being fused. Therefore, filming may occur or external additives used for imparting fluidity might become buried in the inside of the toner. In the electrophotographic process, for keeping and demonstrating the performance of a toner with stability under various types of mechanical stress, it is necessary to inhibit a releasing agent from being exposed in the surface of toner particles, enhance the surface hardness of the toner particles, and increase the smoothness of the surface of the toner particles. The releasing agent may cause various problems if it is exposed in the surface of toner particles. However, taking the performance of the toner at the time of its fixation into consideration, it is preferable that the releasing agent be present near the surface of the toner particles.
With a recent increase of demand for improvement in image quality, there is a remarkable tendency to reduce the size of toner in order to realize highly precise images particularly in color image formation. However, only reducing the size of toner while making the toner have a particle size distribution the same as that of conventional toners have will cause serious problems of contamination of carriers and photosensitive bodies and scatter of toners because of the presence of toners in the fine particle size region in the particle size distribution. It therefore is difficult to realize a high image quality and a high reliability simultaneously. In order to realize a high image quality and a high reliability simultaneously, it becomes necessary to sharpen the particle size distribution of toners and also to reduce the size of toners.
In addition, for the purposes of improvement in color reproduction and expansion of color gamut of color toner images, various kinds of pigments and dyes have heretofore been examined as colorants. However, dyes are inferior to pigments in water resistance and light fastness and may cause problems of color migration and the like when the dyes come into contact with a polyvinyl chloride sheet. Accordingly, pigments are often chosen as colorants for color toners. On the other hand, pigments have drawbacks of poor brightness and poor saturation in comparison with dyes. Therefore, pigments having broader brightness and saturation are always demanded.
Although quinacridone colorants, monoazo colorants, diketopyrrolopyrrole colorants, thioindigo colorants and the like have conventionally been employed for magenta toners, quinacridone pigments have been employed widely because they are superior in sharpness and transparency.
However, toners made from quinacridone colorants are characterized by their bluish tint, which is strong relative to the hue, brightness and saturation of the magenta defined in Japan Color.
On the other hand, toners made from monoazo colorants are characterized by their yellowish tint, which is strong relative to the hue, brightness and saturation of the magenta defined in Japan Color.
The importance of color matching between images on a display, printed images and images outputted by an image-forming apparatus, is increasing with recent remarkable progress to DTP.
An approach for improving the color reproduction, gradation, light fastness and chargeability and for improving the matching with image-forming apparatuses by mixing a quinacridone colorant and a monoazo colorant while making use of their characteristics has been proposed (see JP-A No. 2002-156795).
However, mixing pigments generally causes “cloudiness”, which will result in a problem that a toner made from mixed crystals will have a color gamut narrower than that calculated from the color gamuts of toners each of which is made from a single pigment.